Surface states on amorphous silicon

Pulse measurements on the interface between hydrogenated amorphous Si films and an electrolyte are reported. Such measurements yield the variation of the space-charge density at the amorphous Si surface as a function of barrier height. The high-grade films studied are of large resistivity and the measurements are feasible only if the resistivity is reduced by illumination. In a previous publication we studied depletion layers, where the measurements yielded some information on occupied bulk states in the lower part of the forbidden gap. In this paper we concentrate on accumulation layers, in order to derive the characteristics of unoccupied states in the upper part of the gap. We show that except in very strong accumulation layers, surface states dominate the induced space-charge layer, the contribution of bulk states being insignificant throughout. On an untreated film we find that there is an oxide layer about 3 Å thick. The total surface-state density on such films is around 10¹³ cm^-2. The states are spread out in energy over ∼ 0.3 eV and centered about 0.3 eV below the conduction band edge. Treatment of the film's surface with concentrated HF and immediately immersing the sample into the electrolyte containing 1% HF, results in an oxide-free surface. The surface-state density, however, is not affected significantly. This is in marked contrast with the situation in crystalline Si surfaces, where addition of a minute amount of HF to the electrolyte reduces the surface-state density by one to two orders of magnitude. It appears, then, that the origin of the surface states in amorphous Si is different from that in crystalline Si.